Ozone layer
Ozone is a gas that occurs naturally in our atmosphere. Most of it is concentrated in the ozone layer, a region located in the stratosphere several kilometers above the surface of the Earth. Although ozone represents only a small fraction of the gas present in the atmosphere, it plays a vital role by shielding humans and other life from harmful ultraviolet light from the Sun. Ozone is also a greenhouse gas in the upper atmosphere and, therefore, plays a role in Earth's climate. The increases in primary greenhouse gases, such as carbon dioxide, may affect how the ozone layer recovers in coming years.
The ozone layer refers to the ozone within stratosphere, where over 90% of the earth's ozone resides. Ozone is an irritating, corrosive, colorless gas with a smell something like burning electrical wiring.
The ozone layer in the upper atmosphere protects humans and other organisms from the solar UV radiation. An overexposure to UV radiation leads to a number of serious health risks for humans. It does not only cause sunburns but causes furthermore greater incidences of skin cancer and eye cataracts. Children and light skinned people are particularly vulnerable. There are also serious impacts for biodiversity. For example, increased UV radiation reduces the levels of plankton in the oceans and subsequently diminish fish stocks. It can also have adverse effects on plant growth, thus reducing agricultural productivity. A direct negative economic impact is the reduced lifespan of certain materials like plastics.
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[edit] Ultraviolet radiation
Ultraviolet(UV) radiation is divided into three categories, based on its wavelength; these are referred to as UV-A (400–315 nm), UV-B (315–280 nm), and UV-C (280–100 nm). UV-C, which would be very harmful to all living things, is entirely screened out by ozone at around 35 kilometres (115,000 ft) altitude. UV-B radiation can be harmful to the skin and is the main cause of sunburn; excessive exposure can also cause genetic damage, resulting in problems such as skin cancer. The ozone layer is very effective at screening out UV-B; for radiation with a wavelength of 290 nm, the intensity at the top of the atmosphere is 350 million times stronger than at the Earth's surface. Nevertheless, some UV-B reaches the surface. Most UV-A reaches the surface; this radiation is significantly less harmful, although it can potentially cause genetic damages.
[edit] Ozone Depletion
- Main Article: Chlorofluorocarbon
Gases contain chlorine and bromine atoms, which are known to be harmful to the ozone layer. Important examples are the CFCs and hydrochlorofluorocarbons (HCFCs), human-produced gases once used in almost all refrigeration and air conditioning systems. These gases eventually reach the stratosphere, where they are broken apart to release ozone-depleting chlorine atoms. Other examples are the halons,which are used in fire extinguishers and which contain ozone-depleting bromine atoms.
[edit] Factors affecting Ozone concentrations
- Stratospheric sulfate aerosols: large explosive volcanoes are able to place a significant amount of aerosols into the lower stratosphere, as well as some chlorine. The 1991 Mt. Pinatubo eruption in the Philippines linked to a 20% increase in the ozone hole that following spring.
- Stratospheric winds: Move ozone, not destroy it, the loss of one latitude is the gain of another and globally the effects cancel out.
- Greenhouse gases: to the degree that greenhouse gases might heat the planet and alter weather patterns, the magnatude of the stratospheric winds will certainly be affected.
- Sunspot cycle: ozone is created by solar UV radiation. The amount of UV radiation produced by the sun is not constant but varies by several percent in a rougly 11year cycle.
- Stratospheric chlorine, coming mostly from man-made halocarbons. Careful subtracting of other natural factors yields a net decrease of 3% per decade in global ozone,1978-1991; due most likely to catalytic degredation by stratospheric chlorine.
[edit] Consequences of ozone layer depletion
- Increased UV. Ozone, while a minority constituent in the Earth's atmosphere, is responsible for most of the absorption of UVB radiation. The amount of UVB radiation that penetrates through the ozone layer decreases exponentially with the slant-path thickness/density of the layer. Correspondingly, a decrease in atmospheric ozone is expected to give rise to significantly increased levels of UVB near the surface.
- Effects on humans. The most common forms of skin cancer in humans, basal and squamous cell carcinomas, have been strongly linked to UVB exposure. The mechanism by which UVB induces these cancers is well understood—absorption of UVB radiation causes the pyrimidine bases in the DNA molecule to form dimers, resulting in transcription errors when the DNA replicates. Another form of skin cancer, malignant melanoma, is much less common but far more dangerous, being lethal in about 15–20% of the cases diagnosed. The relationship between malignant melanoma and ultraviolet exposure is not yet well understood, but it appears that both UVB and UVA are involved. Studies are suggestive of an association between ocular cortical cataracts and UV-B exposure, using crude approximations of exposure and various cataract assessment techniques.
- Effects on crops. An increase of UV radiation would be expected to affect crops. A number of economically important species of plants, such as rice, depend on cyanobacteria residing on their roots for the retention of nitrogen. Cyanobacteria are sensitive to UV light and they would be affected by its increase.
